In the field of printing machines, in particular for securities such a banknotes, checks and other similar printed matter, many quality controls are carried out during the entire printing process. Indeed, it is very important to ensure a high quality of production, especially in the field of securities, and hence the precise controls. Such controls can take the form of a check of the quality of the printing, of the recto-verso register of prints on the sheet of substrate etc. It is of course also necessary to control the shape of the substrate, i.e. to check that the substrate, for example a paper substrate, is not torn or folded.
In known methods and devices of the prior art, the quality control of a paper substrate during the printing process is made by a tactile process by contacting the entire surface of the sheet of paper. Such a method has several drawbacks, i.e. the fact that it contacts the substrate and also the fact that is has to be able to follow the speed of the sheets being controlled. Moreover, a direct contact with the substrate may mark or damage said substrate, causing defects in the printing process.
Other devices use photocells to control the sheet edge, said cells being triggered by the machine. They are thus speed and paper size dependent.